1. Technical Field of the Invention
The present invention relates to an overheat preventing apparatus for preventing an electric motor from overheating, and particularly to an overheat preventing apparatus suitable for an electric motor mounted on an electric or hybrid vehicle.
2. Description of the Related Art
For preventing overheating of an electric motor mounted on a vehicle such as an electric or hybrid vehicle for generating a driving force of the vehicle or converting kinetic energy of the vehicle into electric energy to charge an electrical storage device, there is known a technique, for example, as disclosed in Japanese patent laid-open applications No. 11-27806 (Patent Document 1) and 2000-32602 (Patent Document 2), which detects the temperature of the electric motor using a temperature sensor to forcibly limit the output power of the electric motor according to the detected temperature. Further, the present applicant has proposed another technique, as disclosed in Japanese patent laid-open application No. 2002-369578, or equivalent U.S. Pat. No. 6,636,788 (Patent Document 3), for estimating temperature changes of an electric motor without using a temperature sensor to limit the output power of the electric motor according to the estimate.
According to Patent Document 1, a future temperature reaching point is estimated based on a history of detected temperature values so that the output power of the electric motor will be limited not to allow the estimated temperature to exceed a predetermined value, and hence to prevent overheating of the electric motor. According to Patent Document 2, the temperature at which the limiting of output power of the electric motor starts is set lower in a regenerative mode (when the electric motor regenerates electric power) than in a power mode (when the electric motor generates a driving force of the vehicle) to reduce the heat generation of the electric motor. According to Patent Document 3, when an estimated temperature change (temperature rise) exceeds a predetermined value, the output power of the electric motor is limited to prevent overheating of the electric motor.
As mentioned above, the structures disclosed in Patent Documents 1 to 3 are all to limit the output power of the electric motor as appropriate to reduce the heat generation of the electric motor, and hence to prevent overheating of the electric motor. However, when the vehicle is going up a hill or carries a heavy load, the aforementioned structures may make it difficult to produce torque enough to deliver performance required for the vehicle. Such a problem is likely to arise especially with electric vehicles having only the electric motor as their power source.
It is therefore desirable to avoid limiting the output power of the electric motor as much as possible. An alternative structure can be considered, which supplies a coolant to the electric motor to actively cool the electric motor. In this case, however, if the coolant is supplied improperly, the amount of energy consumed by a mechanism such as a pump for supplying the coolant may become unnecessarily too large to provide energy savings.